It is well known that sulfur-containing salts of alkali metals, such as, mainly, Na.sub.2 S, NaHSO.sub.3, Na.sub.2 SO.sub.3, NaHSO.sub.4, Na.sub.2 SO.sub.4, are classified among the indispensable industrial products.
The yearly consumption of such compounds is of the order of millions of tons worldwide.
The production of these salts mostly derives, either directly or indirectly, from sodium chloride, through the production of NaOH or of Na.sub.2 CO.sub.3, by means of well-known electrochemical or chemical processes.
A large amount of these sulfur-containing alkali metal salts, sometimes larger than the demand, is also available as by-products from several basic industries, such as, e.g., sulfur removal from combustion exhaust gases, petrochemical industry, organic syntheses, man-made fibers, wood industries, recovery of lead batteries, and so forth.
Re-processing these by-products, in order to obtain pure salts to be returned to the manufacturing cycle, not always is advantageous, above all when the salts to be recovered are in the form of more or less concentrated aqueous solutions, and as mixtures with each other.
The concentration and separation costs are often already high enough to void the advantages of the reclaiming operation.
On the other hand, no solutions of these salts, even when diluted, may be disposed of without infringing the environmental laws, unless they are preliminarily submitted to expensive purification or recovery processes.
As mentioned above, such sulfur-containing alkali metal salts mostly derive, other directly or indirectly, from caustic soda.
It is well known that caustic soda is nearly exclusively manufactured by means of sodium chloride electrolysis processes, which also imply the production of the corresponding amount of chlorine gas.
The simultaneous production of both products creates market imbalances when the demands for NaOH and Cl.sub.2 are different.
In this regard, it should be observed that in the past years, chlorine demand was larger than for caustic soda, and the latter was regarded nearly as a by-product. During the last few years (and for future years), as a consequence of chlorinated organic solvents banning, the situation is now reversed; at present, caustic soda can be regarded to be the main product; this is mirrored by the international price of this commodity, which has practically doubled over the past five years.
In order to satisfy the increasing demand for NaOH, during the past years, electrodialysis processes were developed in order to produce caustic soda and diluted sulfuric acid from sodium sulfate.
The electrochemical techniques to accomplish this process are sophisticated, both because they require the use of cells with 3 or 4 compartments, as well as owing to the need, due to reasons of energy costs, for depolarizing the anodic reaction with the same hydrogen produced at cathode, by means of the use of hydrogen diffusion anodes, similar to those as used in fuel cells.
Such recent technological solutions, besides being complex, imply the further problem that by-product sulfuric acid is formed, which is formed, which is known to be difficultly traded, also at very low prices.